A phenomenon called “pre-ignition” sometimes occurs in an internal combustion engine. Pre-ignition occurs when an air-fuel mixture in a cylinder ignites prior to the intended ignition by a spark plug. When pre-ignition occurs, an abnormally high in-cylinder pressure is generated, and there is a risk of the engine being damaged. There is also the problem that knocking is induced by the pre-ignition and noise is generated.
Ions are generated when pre-ignition occurs. If the ions reach the electrodes of a spark plug, an ion current flows between the electrodes of the spark plug due to the ions. An apparatus disclosed in Patent Literature 1 that is mentioned below detects the aforementioned ion current and thereby detects the occurrence of pre-ignition. According to Patent Literature 1, it is assumed that pre-ignition occurs due to heat of the spark plug itself. With the apparatus disclosed in Patent Literature 1, when occurrence of pre-ignition is detected, fuel injection is performed in the vicinity of top dead center of an intake stroke so that fuel reaches the spark plug. More specifically, fuel injection is performed so that fuel that has been injected from an in-cylinder injector strikes against the top face of the piston so that the direction of the fuel is changed towards the spark plug side and the fuel reaches the spark plug. The pre-ignition is suppressed as a result of the temperature of the spark plug being lowered by heat of vaporization of the fuel that has reached the spark plug.
Patent Literature 2 that is mentioned below discloses technology that calculates a compression pressure ratio (ΔPT/ΔP0) between a compression pressure (PT) at top dead center of the crank angle and a reference in-cylinder pressure (P0) during a compression stroke as well as an in-cylinder pressure change rate (ΔP/Δθ) with respect to a crank angle (θ) of the in-cylinder pressure (P), and uses these values to determine whether or not pre-ignition and knocking is occurring. When only pre-ignition is occurring, the air-fuel ratio of the engine is increased, and when both pre-ignition and knocking are occurring the air-fuel ratio of the engine is increased and a timing for starting fuel injection is also delayed.
Patent Literature 3 that is mentioned below discloses technology that detects pre-ignition by comparing an in-cylinder pressure that is predicted by a physical model and an in-cylinder pressure that is measured by an ha-cylinder pressure sensor, and suppresses pre-ignition by injecting fuel again from an in-cylinder injector when pre-ignition has been detected.